There are various types of antenna systems mounted on a vehicle nowadays. For example, radio receivers, television receivers, portable telephone systems, GPS (Global Positioning System), ETC (Electronic Toll Collection System), VICS, etc. have their own antenna systems that fit to their specific operation. Since vehicles are mobile substance, it is not easy for them to recognize direction of a certain signal where it is coming from, with these exceptions of GPS, ETC, etc. where recognition of the signal direction is comparatively easy. Based on the general understanding, radiation pattern of antenna for vehicles other than that for GPS, ETC, etc. has been designed to be non-directional with respect to horizontal direction of a vehicle.
Japanese Patent Unexamined Publication No. H8-298406 (hereinafter referred to as Document 1), Utility Model Unexamined Publication No. S58-61509 (Document 2) and Japanese Patent No. 3594224 (Document 3) are some of the known publications of prior arts on the on-vehicle antenna systems.
FIG. 22 shows a typical example of the on-vehicle antenna system disclosed in Document 1. Illustrated in FIG. 22 are first antenna wire 1001, second antenna wire 1002, power supply point 1003 provided for connection with the inner conductor of a coaxial cable which leads to a certain receiver unit, and rear window glass 1004 at the side of a vehicle. First antenna wire 1001 and second antenna wire 1002 are formed, respectively, into a rectangular shape, each having longer sides and shorter sides of its own. Between the longer sides of first antenna wire 1001 is a space D, and a space L between the shorter sides. There is a space K between the shorter side of first antenna wire 1001 and the shorter side of second antenna wire 1002. The antenna can be made to exhibit a non-directional characteristic by adjusting the spaces D, L and K.
Document 2 describes an on-vehicle antenna of space diversity antenna system. The antenna aims to make the directional characteristic into a substantially non-directional characteristic through a compensation of dip point of directional characteristic caused by the vehicle body, etc., using a plurality of antennas disposed at the vehicle's side window.
For the purpose of reducing the overall size of antenna system, monopole antennas of imbalanced operation have been employed for receiving television, radio broadcastings. Dipole antennas of balanced operation are not quite popular nowadays because they eventually take a large total size, and some other reasons. Monopole antenna element alone can not operate as an antenna, but it has to make use of metal body of the vehicle and the ground portion of coaxial cable's power supply line, etc. as part of the antenna system.
The antenna described in Document 1 is an imbalanced type antenna, which belongs to the same type as monopole antenna. It makes use of the metal body of vehicle and the ground portion of coaxial cable's power supply line as part of the antenna. Document 3 describes an imbalanced type antenna for use on a vehicle.
So far, on-vehicle antennas for radio, television reception have been designed so as they are non-directional; therefore, those of imbalanced type have been employed. However, as compared with an antenna installed above the roof of a vehicle, the above-described antenna installed at glass portion of vehicle demonstrates the significantly poorer reception characteristics.
FIG. 23 shows structure of a conventional dipole antenna. Distance between power supply section 1005 and base board 1006 is 15 mm, distance between first parallel side 1007 and second parallel side 1008 is 0.1 mm, length of first parallel side 1007 and second parallel side 1008 is 25 mm, length of first base side 1009 and second base side 1010 is 43.25 mm.
Those illustrated in FIG. 24 through FIG. 26 are used for describing the characteristics exhibited by a monopole antenna disposed at a vehicle's front windshield and a monopole antenna installed above the roof board for receiving digital surface wave television broadcasting.
FIG. 24 illustrates places where antenna was installed for receiving digital surface wave television broadcasting. Monopole antenna unit 2 was installed at three places of a sedan-type vehicle body. Installation point P1 is on the rear part of roof board 1, installation point P2 is at the lower area 23 of front windshield 3 on the cabin surface, installation point P3 is at the upper area 22 of front windshield 3 on the cabin surface. The receiving characteristics of the antenna at these three installation points were evaluated on.
FIG. 25 shows structure of monopole antenna unit 2. Monopole antenna unit 2 includes cylindrical antenna element 4 made of a conductive material, circuit board 5 which is mounted with circuit components such as a filter, an LNA (Low Noise Amplifier), etc., and coaxial cable 6 which connects with a tuner. Monopole antenna unit 2 does not operate with cylindrical antenna element 4 alone, but it functions as an antenna with collaboration of a ground plate provided on circuit board 5, a shield wire of power supply cable 6, and a vehicle frame made of conductive material.
FIG. 26 shows average reception power and percentage of reception in receiving a digital surface wave television broadcasting transmitted from a certain transmitting station, during a test conducted in a certain evaluation course which takes about 6 km a round. Shown in the chart is percentage of error-free receiving time, without a packet error, during one round cruising of the evaluation course. The result shown in FIG. 26 tells us that the reception percentage is the highest, although reception power is low in average, when monopole antenna unit 2 is installed above roof board 1, viz. installation point P1, as compared with the other setups where it is installed at the front windshield glass on the cabin surface, viz. installation points P2 and P3. Two other tests conducted in other district about several hundreds kilometers away from the above-mentioned evaluation course affirmed the earlier-demonstrated test result. The vehicle used in the tests is a sedan-type car installed with monopole antenna unit 2. A wagon-type car with the antenna unit also showed the same result. It was further recognized that even in a case where monopole antenna unit 2 was installed at a window glass other than front windshield, for example at a side window glass or rear windshield glass, the reception percentage was higher than the case where it was mounted above roof board 1.
Reasons why receiving characteristics deteriorate when monopole antenna unit 2 is installed at a window glass on the cabin surface, as compared with a case where it is disposed outside the cabin, had not been made sufficiently clear. The engineers involved in the present proposed technology started a thorough analysis of the causes by carrying out a number of experiments and simulations, and tried to find out a solution for improving the deterioration problem. They found out that the deterioration was caused in part by those reflected/scattered waves generated as the result of reflection/scattering of digital broadcasting waves by the vehicle's metal frame.
FIG. 27 shows a change along with the lapse of time with electric intensity of a 470 MHz-770 MHz plane wave incidental from outside of a vehicle and received by a monopole antenna installed on the vehicle's roof board.
FIG. 28 shows time-wise change in electric intensity of the wave received by a monopole antenna installed at upper area 22 of front windshield glass. Characteristics charts of FIG. 27 and FIG. 28 are those made available by a simulation analysis.
FIG. 29 is a time-wise waveform of electric intensity shown by a plane wave incoming from outside of a vehicle. The plane wave is arriving at the vehicle front with an angle of elevation 30 degrees.
As understood from FIG. 27, the electric intensity received by a monopole antenna installed above a vehicle's roof board shows a waveform pattern which is similar to that of incidental plane wave shown in FIG. 29. Waves reflected/scattered by a vehicle's metal frame are hardly observed received. On the other hand, the chart of electric intensity received by a monopole antenna disposed on upper area 22 of windshield shown in FIG. 28 indicates that the waves reflected/scattered by vehicle body, etc. are reaching the antenna with a delay of approximately 15 ns after arrival of direct wave 7. The approximate delay time 15 ns corresponds to a time which is needed by an electromagnetic wave to proceed for 4.5 m, or a time needed by an electromagnetic wave to go and return inside of the model vehicle cabin which was used in the present simulation analysis.
Judging from the results of experiments and simulations, a monopole antenna disposed at a vehicle's glass portion receives a number of those waves reflected/scattered by the vehicle's metal frame, etc.
The results of simulation analysis shown in FIG. 27 through FIG. 29 represent those situations where only one wave signal is arriving at a vehicle from the outside. In reality, however, a monopole antenna receives quite many signals at the same time, including those reflected/diffracted by buildings and other substances. Each of these signals is reflected/scattered by the vehicle's metal frame, and the monopole antenna receives also such reflected/scattered waves. These incoming waves change from time to time depending on changes in the environmental conditions for an electromagnetic wave, namely the change in location of reflecting substance (vehicles, human beings, trees, etc.). Furthermore, since these signals are received by a moving vehicle, the number and the incoming direction of arriving signals change remarkably from time to time. When an antenna receives a substantial number of such reflected/scattered waves that is changing moment after moment, it becomes difficult to conduct an equalization processing on propagation path at signal demodulation. Therefore, it makes it difficult to realize a high reception percentage, as shown in FIG. 26, despite the high average receiving power. The equalization processing of propagation path, which is a well-known technology among those in the industry, is for restoring a symbol's amplitude/phase information, which changes depending on a state of propagation path, to the original orientation based on information from a pilot signal.
Other deterioration factor with the reception percentage due to reflected/scattered waves in a vehicle cabin is that there is a difference in the Doppler frequency between a signal coming from the front or the behind of a vehicle received direct by on-vehicle antenna system and that received after it is reflected/scattered in the vehicle cabin. When a plurality of signals each having different Doppler frequency undergo a synchronized detection, symbol location of each demodulated signal is displaced along with the lapse of time from a should-be location, because of influence by the Doppler frequency. Especially in the digital television broadcasting which adopts OFDM (Orthogonal Frequency Division Multiplex) modulation, interference is caused at the synchronized detection between the carriers by the reflected/scattered waves in the cabin. Because of these, it turns out to be difficult to enforce the equalization processing on propagation path at a high accuracy level. This invites deterioration in bit error rate (BER) and packet error rate (PER), eventually causing deterioration of the antenna's receiving characteristics. The adverse influence of those waves reflected/scattered in a vehicle cabin reveals significantly when receiving the digital television broadcasting, digital radio broadcasting and portable telephone system which use digital signals. The influence ill-affects the demodulation also with the analog radio broadcasting and analog television broadcasting which use analog signals, and deteriorates the reception characteristics.